1. Field of the Invention
The present invention relates to a high density array of very small dimensional semiconductor devices electrically isolated one from the other with an improved field shield structure. The invention further relates to a method of manufacturing such an array of semiconductor devices.
2. Description of the Prior Art
Using LOCOS (local oxidation of silicon) is a typical method for electrically isolating very small dimensional semiconductor devices arrayed in a high density. However, LOCOS is not adequate when the semiconductor devices are to be isolated at an extremely short-interval therebetween, because the oxidized silicon layer formed by means of LOCOS method tends to penetrate into an adjoining active region existing in the substrate. This phenomenon is known as bird's beak. Recently, the field shield isolation technique has been receiving expert's attention and is considered to be replaced with LOCOS.
Principle of the field shield isolation will be briefly described with reference to FIG. 1 in which is shown an N-channel MOSFET isolated with a field shield electrode. In a P-well or P-type substrate 1 is formed an N-channel MOSFET having a source derived from an N-type impurity diffusion layer 2, a drain derived from the same type impurity diffusion layer 3, and a gate portion 10 having a gate electrode 5 disposed on an insulation layer 4 and a spacer 8. Such a MOSFET is electrically isolated from adjacent semiconductor device with a MOS structure made up of a field shield electrode 6, a field shield gate insulation layer 7 and the P-well or P-type substrate.
Since the field shield electrode 6 is grounded (0 volt), electrons are not induced in the surface of the substrate 1 beneath the field shield electrode 6. Therefore, electrical isolation of the MOSFET can be achieved. However, one of disadvantages with such a structure of the field shield electrode 6 is that when an interval of the isolation is as short as a sub-micron, a leak current is produced due to short-channel effect.